The present invention relates to a radiation detector, an array of a plurality of radiation detectors and a manufacturing method for manufacturing a radiation detector, wherein the radiation detector may, for example, be a bolometer.
Uncooled resistance microbolometer (hereinafter denoted as microbolometer) arrays may be used for detecting far infrared radiation (8 μm-14 μm) and are also included within the term infrared focal plane arrays (IRFPA). Individual microbolometers, as shown in FIG. 11, may be characterized by a membrane 10, which is suspended in a vacuum over the substrate 20 by two metal contacts 15 by means of thin ridges and is thermally insulated by the ridges 22 (shown in FIG. 11). Essentially the membrane consists of an absorber 25 and a sensor layer 30. To ensure a lowest possible reflection of the incident infrared radiation, the layer resistance of the absorber layer 25 may be adapted to the wave resistance of vacuum (about 377 Ohm/sq). Furthermore, below the membrane there may be a metal layer (reflector) 35 on the substrate 20, whereby the partly transmitted radiation is reflected back and subsequently absorbed again by the upper absorber layer. The cavity 40 between the absorber layer 25 and the lower reflector forms an optical resonator. To fulfill the resonator condition (Eq. 1), the distance may be chosen so that the optical path (nd) is an odd multiple of a quarter of the main wavelength λ that is to be detected. The optical path consists of the sum of the layer thicknesses weighted with the refractive indexes of the media inside the cavity (Eq. 2). For a body having a temperature of, e.g., 300 K, the maximum spectral radiance would be about at λ=10 μm. This results in an optical path of nd=2.5 μm (k=0).
                    nd        =                              (                                          2                ⁢                k                            +              1                        )                    ⁢                      λ            4                    ⁢                      (                                          k                =                0                            ,              1              ,                              2                ⁢                                                                  ⁢                …                                      ⁢                                                  )                                              (                  Eq          .                                          ⁢          1                )                                nd        =                  ∑                                          ⁢                                    n              i                        ⁢                          d              i                                                          (                  Eq          .                                          ⁢          2                )            
Due to the absorption of the incident infrared radiation, the thermally insulated membrane 10 may heat up, which, for example, may result in a change of the electrical resistances of the sensor layer 30. Thereby, the temperature change of the membrane 10 depends on the thermal insulation by means of the ridges 22 and on the energy of the absorbed radiation and is generally smaller than change of the radiator temperature by several magnitudes. The resistance change of the sensor layer may then be determined by means of a read out circuit (ROIC).
A decisive performance indicator for microbolometers is the so-called noise equivalent temperature difference (NETD). This factor is defined as the temperature change of an object that generates a change of the measurement signal, which corresponds to the noise of the system and is therefore a measurement for the sensitivity of the sensor (Eq. 3).
                    NETD        =                                            4              ⁢                              F                2                                                    A              ⁢                                                          ⁢              ɛπ              ⁢                                                d                  ⁢                                                                          ⁢                  L                                                  d                  ⁢                                                                          ⁢                  T                                                              ⁢                                                                                          u                    n                    2                                    _                                            ⁢                              g                th                                                                    U                bias                            ⁢              TCR                                                          (                  Eq          .                                          ⁢          3                )            
F is the f-number, A the absorber area, ε the emission coefficient, L the radiance and T the temperature of the object, un2 the square of the overall noise voltage, gth the thermal conductance and Ubias the bias voltage.
From Eq. 3 it can be seen that the NETD, amongst others, is significantly influenced by the thermal insulation of the membrane and the corresponding thermal conductance gth, respectively. Generally, the membrane is poorly thermally insulated from the substrate by means of only the suspension on the metal contacts. In this case, the resulting thermal conductance is not sufficiently small to achieve a good performance, since the contact tubes consist of thick metal coatings due to process and stability reasons and therefore conduct the resulting heat in the membrane relatively well. The limit for the NETD should be, for example, significantly lower than 100 mK, however, it may be higher (smaller NETD values correspond to a better performance).
In conventional microbolometers, significant improvement of the thermal insulation and reduction of the thermal conductance, respectively, are realized by additional connecting elements, the ridges 22, between the suspended membrane and the metal contacts. The thermal conductance of the ridges may be determined by
                              g          ridges                =                  2          ⁢                                                    b                ridge                            ⁢                              d                ridge                                                    l              ridge                                ⁢                      ∑                                                  ⁢                          λ              i                                                          (                  Eq          .                                          ⁢          4                )            wherein λi is the thermal conductivity of the individual ridge materials, bridge and dridge are the width and thickness of the individual ridge materials and lridge is the length of the ridges. Hence, to achieve good thermal insulation, the cross-sectional area of the ridges should be as small as possible and the ridges should consist of materials having a low thermal conductivity. Regarding the heat insulation, the proportion of the metal contacts is mostly to be neglected compared with that of the ridges. Further, the thermal insulation may be influenced by the heat radiation to the surroundings. However, as the infrared detectors are operated in the vacuum, the influence is mostly very small so that the thermal conductivity of the ridges dominates overall.
The trend in the development of microbolometers is moving towards ever smaller pixel sizes for high resolution IRFPAs with simultaneously increasing requirements in terms of performance. Currently, microbolometer arrays are generally manufactured with a pixel pitch of 17 μm. However, it is foreseeable that a new generation with a pixel pitch of 12 μm will be established over the next years. A scaling of the pixel pitch from 17 μm to 12 μm means halving the absorber area 25. Generally, miniaturization of the pixel pitch due to the reduction of the absorber area 25 may have a massive impact on the performance of the microbolometers.
The effective absorber area 25 may be limited due to the required area for realizing the ridges. Depending on design and structure of the ridges and target value of the thermal conductance, respectively, the occupied area of the ridges 22 may have a varying size. However, beside the thermal conductance, the absorber area 25 may likewise have an impact on the performance. Now, if the pixel area is decreased by a certain factor, the whole microbolometer could theoretically be scaled accordingly so that the proportions of the individual areas (ridges 22, contacts 15, absorber area 25) and distances to each other remain the same. The performance loss would then be determined, amongst others, by the scaling factor. However, the problem regarding scaling is that, here, the boundaries of lithography could be quickly reached. Typically, a stepper lithography with a resolution of 0.35 μm is used for manufacturing microbolometer arrays. Frequently, structure sizes at the limit of this resolution are already used in current but also in older microbolometer generations (17 μm, 25 μm, 35 μm), as, for example, in the ridge widths and distances. On the other hand, also due to process and stability reasons, the contact holes and upper contact areas cannot be scaled arbitrarily small so that a limit exists here as well. Due to these problems, especially the ridge areas may use more and more space relative to the pixel size at a set thermal conductance (specified by concept), whereby the effective absorber area is additionally reduced and the performance is heavily decreased.
In the literature, another arrangement has been presented, wherein the absorber layer is stretched umbrella-like over the entire pixel area. Such an arrangement is called two-layer design or, specifically, umbrella design. Hereby, it is also possible to manufacture bolometers with a large absorber area having, at the same time, good thermal insulation. However, the disadvantage herein is that the ridges and the sensor layer are still in one plane. Thus, the thermal insulation is limited by the free available pixel area. Furthermore, the resonator condition is not fulfilled in the region of the suspension of the absorber, having a negative effect on the absorption. The manufacturing process of two-layer bolometers (ridges and absorber not in one plane) is also significantly more extensive.